Method and apparatus for making crystalline pet pellets

ABSTRACT

A method and apparatus for underwater pelletizing and subsequent drying of polyethylene terephthalate (PET) polymers and other high temperature crystallizing polymeric materials to crystallize the polymer pellets without subsequent heating. High velocity air or other inert gas is injected into the water and pellet slurry line to the dryer near the pelletizer exit. Air is injected into the slurry line at a velocity of from about 100 to about 175 m 3 /hour, or more. Such high-speed air movement forms a vapor mist with the water and significantly increases the speed of the pellets into and out of the dryer such that the PET polymer pellets leave the dryer at a temperature sufficient to self-initiate crystallization within the pellets. A valve mechanism in the slurry line after the gas injection further regulates the pellet residence time and a vibrating conveyor after the dryer helps the pellets to achieve the desired level of crystallinity and to avoid agglomeration.

RELATED APPLICATIONS

This application is a divisional application of application Ser. No.10/954,349, filed Oct. 1, 2004, which is a continuation-in-partapplication of application Ser. No. 10/717,630, filed Nov. 21, 2003, nowU.S. Pat. No. 7,157,032 issued Jan. 2, 2007, and hereby claims thepriority thereof to which it is entitled.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a method and apparatus forunderwater pelletizing and subsequent drying of polyethyleneterephthalate (PET) polymers. More specifically, the present inventionrelates to a method and apparatus for underwater pelletizing PETpolymers and subsequent drying of the PET polymer pellets in a manner toself-initiate the crystallization process of the PET particles andproduce pellets having a desired level of crystalline structure ratherthan an amorphous structure.

2. Description of the Prior Art

Underwater pelletizing systems for producing pellets of polymeric orother plastic materials has been known for many years. The startingmaterials such as plastic polymers, coloring agents, additives, fillersand reinforcing agents, and modifiers, are mixed in kneaders. In theprocess, a melt is produced which is extruded or pressed through dies toform strands which are immediately cut by rotating cutter blades in thewater box of the underwater pelletizer. Water with or without additivesis continuously flowing through the water box to cool and solidify thepolymer strands and pellets and carry the pellets out of the water boxthrough transport piping to a dryer, such as a centrifugal dryer, wherethe water is removed from the pellets.

For quite some time, the polymer industry has sought to process PETpolymers into a pellet shape using underwater pelletizer systems. Amajor drawback of using underwater pelletizing, as well as otherpelletizing systems, for processing PET into pellet shapes is thetypically amorphous condition of these pellets when they leave the dryerof the underwater pelletizing system. The amorphous nature of theresulting pellet is caused by the fast cooling of the PET material onceintroduced into the water flow in the water box of underwater pelletizerand while the water and pellet slurry is being transported byappropriate piping to the dryer.

Typically, increasing the water flow through the water box of theunderwater pelletizer and increasing the water temperature, along withpipe dimensional changes and reducing the distance between thepelletizer and dryer unit, does not help to sufficiently maintain thepellet temperature. Under such circumstances, the PET pellets stillleave the dryer at a temperature, usually below 100° C., which is belowthe temperature at which crystallization can occur.

End users of PET polymer pellets typically require that the pellets bein a crystalline state, rather than an amorphous state, principally fortwo reasons, both relating to the fact that the end user wants toprocess the PET pellets in a substantially dry condition, with zero ornear zero water content. First, PET polymers are very hygroscopic, andcrystalline PET pellets absorb considerably less moisture duringshipment and storage than amorphous PET pellets. Accordingly,crystalline PET pellets can be dried to the requisite zero or near zeromoisture content more easily by the end user. Second, the temperaturerequired to completely dry PET polymers is higher than the temperatureat which amorphous PET pellets convert to the crystalline form.Therefore, when drying amorphous PET pellets, it is necessary to firstachieve crystallization at the requisite lower temperature beforeraising the temperature to the drying temperature. Otherwise, theamorphous PET polymer pellets may agglomerate and destroy the pelletform.

As a result, manufacturers of PET pellets must typically subject theamorphous PET pellets to a secondary heating step of several hours atvery high temperatures, usually in excess of 80 to 100° C., to changethe amorphous structure of the pellets to a crystalline structure. Thisis a very expensive second step in order to convert the PET polymerpellets into the desired crystalline state.

However, it is recognized by the end users and manufacturers of PETpellets that total (100%) crystallinity of the PET pellets is notnecessarily required in order to dry the PET pellets for furtherprocessing or use in the Solid State Process (SSP). Rather, a totalcrystallinity, or crystallinity grade using the Calcium Nitratemeasurement method, above 30%, and preferably above 40%, is acceptablefor the PET end users.

An alternative approach is disclosed in WO 2004/033174 in which thepolymer is granulated or pelletized in a water bath at a temperature ofmore than 100° C. The resulting pellets may be further treated in thewater bath for a defined period of time thereafter, while retaining thehigh temperature, in order to convert the amorphous material into acrystalline material. This system requires pressurization to maintainthe water at the super-boiling point temperature, followed by a pressurereduction procedure.

It is also known generally that air can be injected into the exit streamof a water and pellet slurry from a pelletizer in order to enhance thetransport of the water/pellet slurry. See, for example, U.S. Pat. No.3,988,085.

SUMMARY OF THE INVENTION

The present invention is directed to an underwater pelletizing systemthat produces PET pellets in a hot enough condition to self-initiate thecrystallization process therein and ultimately provide a sufficientlycrystalline character such that the PET pellets do not require aseparate heating step in order to undergo end user processing. It hasbeen discovered that this elevated heat condition can be accomplished byreducing the residence time of the pellets in the water slurry in orderto leave enough heat in the PET pellets during the drying stage so thatthe crystallization process is initiated from inside the pellets. To dothis, it is necessary to separate the pellets from the water as soon aspossible and to significantly increase the speed of pellet flow from theexit of the underwater pelletizer and into and through the dryer. Thehot pellets leaving the dryer can then be carried on a conventionalvibrating conveyor or other vibrating or handling equipment for a timesufficient to achieve the desired crystallinity and avoid agglomeration.The hot pellets can also be stored in a heat retaining condition, suchas in a heat insulating container, to complete the desiredcrystallization process. For example, coated steel or plastic containersshould be acceptable, instead of the stainless steel boxesconventionally used.

The early pellet/water separation and increased pellet speed through thepelletizer system is accomplished in accordance with the presentinvention by injecting air or other suitable gas into the transportationpiping leading from the pelletizer to the dryer just after the cutpellets and water slurry exit the water box of the pelletizer unit. Ithas been found that the injected air serves to separate the water fromthe pellets in the transportation piping by converting the water to awater vapor mist, significantly speeds up the transport of the pelletsto and through the dryer, and can serve to generate a pellet temperatureexiting the dryer that is sufficiently high to initiate thecrystallization process within the pellets. Specifically, while the PETpolymer pellets may come out of the dryer in an amorphous condition,there is still sufficient heat remaining inside the pellets forcrystallization to occur. The extent of the crystallization issufficient to eliminate the necessity of the second heating stageheretofore required to make PET pellets using previous underwaterpelletizing systems.

The air introduced into the slurry line leading to the dryer immediatelyafter the exit from the water tank is at a very high velocity. It hasbeen found that an air volume of from at least 100 cubic meters(m³)/hour, to about 175 m³/hour, or more, through a valve at a pressureof 8 bar and into a 1.5 inch slurry pipe line produces the requisite airvelocity for the present invention. The volume of air introduced intothe exiting water and pellet slurry produces an overall gas/slurrymixture in the nature of a mist and is likely to have a gas component of98%-99% or more by volume of the overall mixture. The air injection intothe slurry line dramatically increases the speed of the pellet flow fromthe water box to the exit of the dryer to a rate less than one second.While air is the preferred gas in view of its inert nature and readyavailability, other inert gases such as nitrogen or similar gases couldbe used. Other pellet speed expediting methods that would comparablyseparate the liquid water from the pellets and accelerate the pelletsfrom the pelletizer to the dryer exit might also be employed.

The slurry piping preferably includes a ball valve or other valvemechanism after the air injection point. The ball valve allows theoperator to better regulate the residence time of the pellets in thepiping and dryer, and serves to significantly reduce or eliminate anyvibrations in the slurry pipe to the dryer. The ball valve or valvemechanism also appears to provide an improved water vapor mist conditionin the slurry pipe downstream of the valve mechanism.

It has been found that crystalline PET pellets can be formed inaccordance with the method and apparatus of the present invention if theresidence time of the pellets from the point of formation by the cutterblades at the die face to the exit from the centrifugal dryer issufficiently reduced by the injection of high velocity air or other gasinto the slurry line. While larger pellets lose their heat more slowlyso as to retain a high enough temperature upon exit to undergocrystallization at lower injected air velocities, such as 100 m³/hour,as the air velocity increases smaller pellets with a lower exittemperatures also exhibit acceptable levels of crystallization. Hence,the rapid separation of the pellets from the water and the shortenedresidence time assures that the PET pellets exit the dryer of theunderwater pelletizing system while retaining sufficient heat inside thepellets to achieve the desired crystallization in the amorphous pellets,particularly if the pellets are transported from the dryer by aheat-retaining vibrating conveyor for a time sufficient to achieve thedesired level of crystallinity, and/or properly stored in a heatinsulating container. As a result, the necessity of a secondary heatingstep is eliminated.

When transported away from the dryer in a vibrating conveyor, it hasbeen found that transport for a time from about 20 seconds to about 90seconds, or more, is sufficient to achieve the desired crystallinity.The preferred transport time is about 30 second to 60 seconds, and themost preferred is about 40 seconds.

Accordingly, it is an object of the present invention to provide amethod and apparatus for processing PET polymers in an underwaterpelletizing system which can produce crystallization in the PET pelletsthat exit from the dryer.

It is another object of the present invention to provide a method andapparatus for producing crystallization in PET polymer pellets utilizingan underwater pelletizing system without the necessity of an expensivesecondary heating stage to convert amorphous PET pellets to crystallinePET pellets.

It is a further object of the present invention to provide a method andapparatus for the underwater pelletizing of PET polymer in which aninert gas is injected into the water and pellet slurry exiting thepelletizer to produce a water vapor mist form of slurry handling,thereby providing better heat retention in the transported pellets.

A still further object of the present invention is to provide a methodand apparatus for underwater pelletizing of PET polymer in accordancewith the preceding object in which the pellets are rapidly transportedthrough the equipment through the injection of air at a velocity of atleast 100 m³/hour, to about 175 m³/hour or more, so that the residencetime of the pellets before exiting the dryer is sufficiently reduced togenerate crystallization on the order of 30%-40% of total (100%)crystallization.

It is yet another object of the present invention to provide a methodand apparatus for producing PET polymer pellets using an underwaterpelletizing system in which the pellets exiting the dryer havesufficient heat remaining inside the pellets for at least 35% totalcrystallization of the PET pellets to occur without subsequent heating.

It is still a further object of the present invention to provide anunderwater pelletizing method and apparatus for producing PET pellets inwhich the residence time of the PET pellets from the time of extrusionat the die face until exit from the centrifugal dryer is reduced to lessthan about one second by gas injection into the slurry line from thepelletizer to the dryer.

A still further object of the present invention is to provide anunderwater pelletizing method and apparatus for producing PET pellets inaccordance with the preceding object in which the residence time isregulated using a valve mechanism for improved pressurization of thewater vapor mist downstream of the valve in the slurry line.

It is another object of the present invention to provide an underwaterpelletizing system in which the hot pellets exiting the dryer arecarried on a vibrating conveyor or other vibrating or handling equipmentto achieve virtually uniform crystallization throughout a given outputpellet volume.

These together with other objects and advantages which will becomesubsequently apparent reside in the details of construction andoperation of the invention as more fully hereinafter described andclaimed, reference being had to the accompanying drawings forming a parthereof, wherein like numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an underwater pelletizing system,including an underwater pelletizer and centrifugal dryer as manufacturedand sold by Gala Industries, Inc. (“Gala”) of Eagle Rock, Va., with airinjection and vibrating conveyor in accordance with the presentinvention.

FIGS. 2A and 2B are schematic illustrations of side and end views,respectively, of the vibrating conveyor of FIG. 1.

FIG. 3 illustrates certain components of the underwater pelletizingsystem shown in FIG. 1 during a bypass mode when the process line hasbeen shut down.

FIG. 4 is a schematic illustration showing a preferred method andapparatus for air (or gas) injection into the slurry line from thepelletizer to the dryer in accordance with the present invention.

FIG. 5 is a schematic illustration showing a preferred method andapparatus for air (or gas) injection into the slurry line from thepelletizer to the dryer with a ball valve in the slurry line, inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Although only preferred embodiments of the invention are explained indetail, it is to be understood that the invention is not limited in itsscope to the details of construction and arrangement of components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments and of being practiced orcarried out in various ways.

Also, in describing the preferred embodiments, terminology will beresorted to for the sake of clarity. It is intended that each termcontemplates its broadest meaning as understood by those skilled in theart and includes all technical equivalents which operate in a similarmanner to accomplish a similar purpose. For example, the term “water”includes not only water itself, but also water with one or moreadditives included, which are added to the water during the underwaterpelletizing step for various purposes used by those skilled in the artof underwater pelletizing.

An underwater pelletizing system for use in association with the presentinvention is schematically shown in FIG. 1 and is generally designatedby reference number 10. The system 10 includes an underwater pelletizer12, such as a Gala underwater pelletizer, with cutter hub and blades 14shown separated from the water box 16 and die plate 18. In theunderwater pelletizing system 10, PET polymer is fed from above from apolymer vat (not shown) into a screen changer 20 which removes any solidparticles or other material. The PET polymer is then fed through gearpump 22 to control and maintain a smooth flow of the polymer into thepolymer diverter 24 and die plate 18. The PET polymer is typicallyextruded through holes in the die plate at a temperature of about 260°C. The PET polymer strands formed by the die holes enter into the waterbox 16 and are cut by the cutter hub and blades 14 into the desiredpellets. Cold water flows into the water box 16 through pipe 26 and thewater and cut pellet slurry exits through pipe 28.

The water and pellet slurry is then conveyed through the slurry line 30into a dryer 32, such as a Gala centrifugal dryer, at inlet 33. Thepellets are dried in the dryer 32 and exit the dryer at 34. The waterremoved from the dried pellets exits the dryer 32 through pipe 38 and isconveyed by pump 40 into a fines removal sieve 42 and thence into awater tank 44 through pipe 46. The recycled water leaves water tank 44through pipe 48 and pump 50 into a water heat exchanger 52 to reduce thewater temperature. The cooled water is recycled through pipe 54 pastbypass valve 56 and pipe 58 to inlet pipe 26 and then into the water box16.

In accordance with the present invention, air is injected into theunderwater pelletizing system in slurry line 30 at point 70, preferablynear the beginning of the slurry line 30 adjacent the exit from thewater box 16, in order to enhance the transport of PET pellets in theslurry line 30 and keep the PET pellets at a high enough temperature tofoster the desired crystallization.

The air is conveniently injected into the slurry line 30 at point 70using a conventional compressed air line typically available in mostmanufacturing facilities, such as with a pneumatic compressor, and astandard ball valve sufficient to produce a high velocity air flow inthe slurry line 30. This is readily achieved by a volume of air of atleast 100 m³/hour through a standard ball valve at a pressure of 8 barinto a slurry line comprising a standard 1.5 inch pipe. This highvelocity air (or other gas) when contacting the water and hot pelletsgenerates a water vapor mist. The pellets tend to disperse to the insidecircumference of the pipe as they move rapidly therethrough to thedryer. It is estimated that the volume of air in the overall gas/slurrymixture is on the order of 98%-99% or more by volume of the overallmixture. The air injected into the slurry line 30 at point 70 increasesthe speed of the pellet flow from the water box 16 to the exit 34 of thedryer 32 to a rate of less than one second.

The mean temperature of the PET polymer pellets exiting the dryer 32 at34 in accordance with the present invention should be above about 145°C. at an air velocity of 100 m³/hour, but may be lower when the airvelocity is increased to 175 m³/hour. With such high velocity pelletspeed expediting action, the PET pellets retain sufficient heat insidethe pellets to initiate crystallization therein, without the necessityof a secondary heating step.

Pellets exiting the dryer are preferably directed through a vibrationunit, such as vibrating conveyor 84, as shown in FIGS. 2A and 2B.Through agitation and mixing of the crystallizing pellets in thevibration conveyor 84, variations in the temperatures of pellets whichmight otherwise occur through proximity of individual pellets to acontainment wall versus immersion amongst other pellets, for instance,are avoided. Instead, uniformity in temperature and in the resultingdegree of crystallization is greatly improved. In addition, stickinessresulting from the elevated pellet temperatures is countered through thejostling and relative movement of the pellets which prevents anyclumping or adherence of the pellets to the surrounding wall structure.

For crystallization purposes, it is has been found that the pelletsshould remain in the vibration conveyor between about 20 and about 90seconds, or more, preferably between about 30 and about 60 seconds, andmost preferably about 40 seconds. During this time, sufficient heat isretained by the vibration conveyor to maintain the pellets at a highenough temperature to complete the desired crystallization. Largerpellets having an exit temperature on the order of 1450 due to theirgreater mass may require only 10 seconds at that temperature withinwhich to achieve 40% crystallization. With their smaller mass andrelatively greater surface area, smaller pellets having a cooler exittemperature of about 127° C. may require 20 seconds at that temperatureto complete the desired crystallization. The remaining time in thevibrating conveyor allows the pellets to cool to a greater or lesserextent.

If additional cooling is required due, for example, to the operator'sinability to store, use or transport heated pellets from the exit of thevibration conveyor, then air blowers may be added at such exit or thevibration conveyor may be designed to provide a residence time of up toapproximately two minutes. Generally, the temperature of the pellets isabout 128° C. at the entrance to the vibration conveyor, and between 60°C. and 110° C. at the exit thereof, depending upon whether or not theoperator has provided for additional pneumatic cooling directly on theconveyor in order to output pellets that are fully cooled for handlingpurposes (60° C.) or instead requires only that the pellets becrystalline (110° C.) upon leaving the vibrating conveyor. The preferredexit temperature for most purposes is less than 80° C., while a highersurface tack temperature (<100° C.) is sufficient for some grades of PETpolymer.

If a vibrating unit is not used, or in addition to the vibrating unit,the PET polymer pellets exiting the dryer 32 can be placed inappropriate heat insulating containers so that the retained heat in thePET pellets is sufficient to complete the desired crystallizationprocess, before the pellets cool below the crystallization temperature.

In by-pass mode shown in FIG. 3, the recycled water goes through bypass56 into pipe 60 and then into slurry line 30. In the bypass mode, thevalve 62 is closed and the water/pellet slurry in line 30 and water box16, along with the water in inlet line 26 can drain from the system outof drain valve 64.

FIG. 4 schematically illustrates one preferred arrangement for airinjection into the slurry line of an underwater pelletizing system inaccordance with the present invention and is generally designated byreference numeral 100. The underwater pelletizer 102 illustrated is aGala Model No. A5 PAC 6, with water inlet pipe 104 and slurry exit line106. The dryer 108 illustrated is a Gala Model No. 12.2 ECLN BF, withthe slurry entrance 110 at the top. Inasmuch as the exit from theunderwater pelletizer 102 into slurry line 106 is significantly belowthe entrance 110 to the centrifugal dryer 108, when both are level on amanufacturing floor, it is necessary to transport the water and pelletslurry upwardly from the pelletizer exit to the dryer entrance. Thewater and pellet slurry thus moves through valve 112 past angled elbow114, through angled slurry line 116, past enlarged elbow 118 and theninto the entrance 110 of dryer 108. The air injection is past nozzle orvalve 120 and directly into the angled elbow 114.

As shown in FIG. 4, the angled slurry line 116 is preferably straightand has an enlarged elbow 118 at its exit end. The enlarged elbowfacilitates the transition of the high velocity water and pellet slurryfrom the straight slurry line 116 into the dryer entrance 110 andreduces potential agglomeration into the dryer 108. Further, the airinjection into the angled elbow 114 is preferably in line with the axisof slurry line 116 to maximize the effect of the air injection on thewater and pellet slurry and to keep constant aspiration of theair/slurry mixture.

While the angle between the vertical axis of slurry line 116 and thelongitudinal axis of angle slurry line 116 is most preferably about 45°,as shown in FIG. 4, a preferred range is 300-600. Moreover, the anglecan be varied from 0° to 90°, and even more in the event the water andpellet slurry exit from pelletizer 102 is higher than the entrance 110to dryer 108 when, for example, the pelletizer and dryer are placed atdifferent levels in the plant or the heights of the components aredifferent than shown in FIG. 4.

With the air injection as described, the residence time of the pelletsfrom the water box to the exit is less than one second which has beenfound to produce pellets with the desired crystallization. However, inanother preferred embodiment, a second ball valve or valve mechanism 150is positioned after the air injection point, as shown in FIG. 5. Thevalve mechanism 150 serves to better regulate the residence time of thepellets within the slurry line while retaining sufficient head pressureon the cutting chamber. This second valve mechanism not only providesfor regulating the residence time of the pellets in the slurry line butalso reduces vibration in the slurry pipe significantly. In addition,the resulting pressurization of the air injected chamber seems toimprove the water vapor mist generated in the slurry pipe downstream,enhancing the results obtained with smaller pellets in particular.

TRIAL EXAMPLES First Trial Set

Molten PET polymer was continuously extruded into an overall underwaterpelletizing system as illustrated in FIG. 1, using a Gala UnderwaterPelletizer Model No. A5 PAC 6 and a Gala Model 12.2 ECLN BF CentrifugalDryer, in the arrangement shown in FIG. 3. The melt temperature wasabout 265° C. and the cutter blade speed in pelletizer 102 was variedbetween 2500 and 4500 RPM. The die plate was typical for PET polymersand a typical 3.5 mm die plate with elongated lands was used. The meltvelocity through the die holes during the trials was constant at 40kg/hole/hr.

The pipe for slurry line 116 was a standard 1.5 inch pipe and its lengthwas 4.5 meters. The speed of centrifugal dryer 108 was kept constantduring the trials, and the countercurrent air flow through the dryer 108was also kept constant during the trials. A vibrating unit was not used.

The air injection flow rate to nozzle or valve 120 was varied from 0 toa maximum of 100 m³/hour, as indicated in Table 1 below, and the waterflow and pellet size also varied, again as indicated in Table 1 below.

The parameters and results of the first set of trials are set forth inTable 1 below.

TABLE 1 Weight of Water Air injection Pellet Pellet a pellet Water -temp rate rate temp Crystallinity Trial size (mm) (g) (° C.) (m³/h)(m³/h) (° C.) grade (%) 1 5.5 × 3.0 0.032 76 13 100 155 98 2 4.5 × 3.00.0299 74 13 100 152 98 3 4.5 × 3.0 0.0306 71 19 0 105 0 4 4.0 × 2.60.0185 64 19 100 130 60 5 3.5 × 3.0 0.0256 69 18 100 136 80 6 4.1 × 3.10.0267 73 18 100 146 98

The pellet temperature and percentage crystallinity as set forth in thelast two columns of Table 1 were determined by examining the productcoming out of the dryer 108 at the end of each trial. Specifically, whenthe pellets were visually inspected it was determined approximately howmany of 100 pellets had undergone a color change indicatingtransformation to a more crystalline state. For example, in trial 5,about 80 out of 100 pellets indicated a color change. Temperature of thepellets was also determined on a surface basis using an infraredtemperature gauge. The extent to which the pellets may have been“totally” crystallized, with “total” crystallization indicating a statein which each pellet is fully crystalline throughout its individualstructure, could not be determined using these external measuringtechniques. However, for practical application the pellets were found tobe sufficiently crystallized for the purposes of PET end users,effectively demonstrating at least 30-40% crystallization duringsubsequent testing, with no need for any additionalheating/crystallizing processing.

At an air injection velocity of 100 m³/hour, it is preferred that 135°C. be the minimum temperature for PET polymer pellets to leave thedryer, when the pellets have the sizes used in the above tests. However,adequate crystallization at lower exit temperatures may be obtained withthis invention if smaller size PET pellets are made, provided the airinjection velocity is increased.

Second Trial Set

Molten PET polymer was continuously extruded into an overall underwaterpelletizing system as illustrated in FIG. 1, using a Gala UnderwaterPelletizer Model No. A5 PAC 6 and a Gala Model 12.2 ECLN BF CentrifugalDryer, in the arrangement shown in FIG. 3. The melt temperature wasabout 265° C. and the cutter blade speed in pelletizer 102 was variedbetween 2500 and 4500 RPM. The die plates used were typical for PETpolymers. In order to be able to work with different pellet sizes, diehole diameters and die hole velocities were varied as well as cutterspeeds.

The pipe for slurry line 116 was a standard 1.5 inch pipe and its lengthwas 4.5 meters. The speed of centrifugal dryer 108 was kept constantduring the trials, and the countercurrent air flow through the dryer 108was also kept constant during the trials. A vibrating conveyor 84 wasused to receive the pellets exiting the dryer.

The air injection flow rate to nozzle or valve 120 was varied from 0 toa maximum of 175 m³/hour, as indicated in Table 2 below, and the waterflow and pellet size also varied, again as indicated in Table 2 below.

The parameters and results of the second set of trials are set forth inTable 2 below.

TABLE 2 Weight Air Amount of A-C of a Water - Water injection Pelletpellets [%] A = Pellet pellet temp rate rate temp amorphous C =Crystallinity Sample size (mm) (g) (° C.) (m³/h) (m³/h) (° C.)crystalline grade (%) 10 3.5 × 2.6 0.015 77 20 175 147 100% C 43.1 112.5 × 3.5 0.015 78 22 0 107 10% C  6.9-30.9 11 3.5 × 2.5 0.015 78 22 0107 90% A 3.5 12 2.7 × 2.7 0.015 78 17 175 129 100% C 43.9 13 2.4 × 3.00.015 78 24 0 109 12% C 10.8-35.6 13 2.4 × 3.0 0.015 78 24 0 109 88% A3.7 14 2.6 × 3.1 0.012 78 22 175 128 100% C 44.1 15 2.6 × 3.1 0.012 7825 0 95 100% A 3.3 16 2.0 × 2.7 0.011 72 20 175 123 100% C 38.9 17 2.4 ×2.4 0.010 75 25 175 117 100% C 43.0 18 2.2 × 2.2 0.008 79 24 175 116 98%C 38.9

Samples 10 and 11 were run under the same conditions except that Sample10 was conducted with air injection at a rate of 175 m³/hour and Sample11 was conducted without any air injection. Similarly, Samples 12 and13, and Samples 14 and 15, were conducted on the same conditions withrespect to each pair, with the exception of the air injection. Samples16, 17 and 18 had no corresponding tests in the absence of air becausethe pellet size was too small for effective processing without airinjection.

From the results in the second trial set, it can clearly be seen thatthe air injection method is essential to maintain a crystalline pellet,specifically when trying to achieve pellet weights below 0.015 g/pelletwhich, in the majority of cases, is the customer target. As compared tothe first trial set which, when summarized in copending application Ser.No. 10/717,630 incorporated herein concluded that a minimum exittemperature was required, the results of the second trial set haveclarified the significance of the air injection velocity to achievingthe desired crystallinity.

The pellet temperature and percentage crystallinity as set forth in thesecond and third to right-most columns of Table 2 were determined byvisual examination and using an infrared temperature gauge, both asdescribed above in connection with the first trial set. Subsequent tothe time at which the first trial set was conducted, however, it wasdetermined that total crystallinity, or crystallinity grade, can bemeasured using the Calcium Nitrate measurement method. The right-mostcolumn shows the results of such an evaluation.

With the air injection method according to the present invention, PETpellets of various sizes can be produced with an acceptablecrystallinity grade. This is even possible with pellet weights as low as0.008 g/pellet provided the air is injected at a high enough velocity.By contrast, using prior art operating devices for pelletizingtechnology including those using extremely short pipe runs and very highwater flows, only a certain percentage, approximately 10-12%, ofcrystalline pellets can be produced. These so-produced pellets, however,contain significant variation in crystallinity from about 6.9% to up to35.6%. This limited degree of homogeneity within the pellets is notacceptable. Furthermore, if the pellet size is reduced to 0.012 g/pelletor below, only by the air injection method of the present invention wasit possible to produce a yield in which 100% of the pellets werecrystallized at least to a 35% grade of crystallinity. PET with acrystallinity percentage of greater than 35% has been found to becrystalline enough for the Solid State Process (SSP) and therefore isacceptable for the PET end users.

As summarized above, the first and second trial sets were conducted withair flow velocities of 100 m³/hour and 175 m³/hour, respectively. Higherair velocities on the order of 200 m³/hour or higher can also be used,as required by water flow and pellet rate changes.

While the present invention is particularly applicable to the underwaterpelletization of PET polymers, it is believed that other polymers whichcrystallize at elevated temperatures and which retain heat whensubjected to high temperatures may also be appropriate for the presentinvention. Such polymers include certain grades of thermoplasticpolyurethane (TPU), PET copolymers and/or PET blends.

The foregoing is considered as illustrative only of the principles ofthe invention. Since numerous modifications and changes will readilyoccur to those skilled in the art, it is not desired to limit theinvention to the exact construction and operation shown and described.Accordingly, all suitable modifications and equivalents may be resortedto, falling within the scope of the invention.

1. An apparatus for processing PET polymers into pellets which comprisesan underwater pelletizer to cut extruded PET polymer strands intopellets, piping to introduce water into said pelletizer and to form aslurry line to transport a water and pellet slurry out of saidpelletizer and to a dryer for drying said PET pellets, and an injectorto introduce a pellet speed expediter into said water and pellet slurryin said slurry line, said injector being configured to inject saidpellet speed expeditor as a gas at a velocity of at least 100 m³/hour,said injector being positioned at an injection point in advance of adownstream portion of the slurry line and before any water is removedfrom said slurry line so that the injection of said pellet speedexpediter increases a speed of water and pellet flow and causes thewater to separate from said pellets while both said water and saidpellets continue to move through said downstream portion of the slurryline, said separation from said water and said increased pellet flowspeed causing said pellets to exit said dryer with sufficient internalheat to initiate crystallization of said pellets.
 2. The apparatus asclaimed in claim 1, wherein the pellet speed expediter is an inert gasmoving at a velocity of between 100-175 m³/hour.
 3. The apparatus asclaimed in claim 1, further comprising an agitation unit that receivespellet output from said dryer and keeps said pellets in movement duringsaid crystallization.
 4. The apparatus as claimed in claim 1, furthercomprising a heat insulating container that receives pellet output fromsaid dryer.
 5. The apparatus as claimed in claim 1, wherein a portion ofsaid slurry line is straight and angled upwardly at an angle between 30°and 60°, said injector being configured to inject said pellet speedexpeditor in line with said straight and upwardly angled slurry lineportion.
 6. The apparatus as claimed in claim 2, wherein said slurryline includes a straight portion and said injector introduces said inertgas at a beginning of said straight portion, and further comprising aball valve serving to regulate residence time of the pellets in saidapparatus.
 7. The apparatus as claimed in claim 6, wherein said injectorintroduces said inert gas into said water and pellet slurrysubstantially in alignment with a longitudinal axis of said slurry linestraight portion.
 8. The apparatus as claimed in claim 1, furthercomprising a valve mechanism downstream of said injector to furtherregulate said pellet speed.
 9. The apparatus as claimed in claim 1,wherein said dryer is a centrifugal dryer configured such that PETpellets exit said dryer at a mean temperature above about 135° C. 10.The apparatus as claimed in claim 1, wherein said dryer is a centrifugaldryer configured such that PET pellets exit said dryer at a meantemperature above about 125° C.
 11. The apparatus as claimed in claim 1,wherein said injector is configured to inject said pellet speedexpeditor as a gas at a velocity of at least 175 m³/hour.
 12. Theapparatus as claimed in claim 1, wherein said pellet speed expeditor ispressurized air at a velocity of between 100-175 m³/hour.
 13. Theapparatus as claimed in claim 12, wherein said injector is configured tointroduce said pressurized air into said water and pellet slurrysubstantially in alignment with a direction of said slurry flow.
 14. Anapparatus for processing polymers into pellets which comprises anunderwater pelletizer to cut extruded PET polymer strands into pellets,piping to introduce water into said pelletizer and to form a slurry lineto transport a water and pellet slurry out of said pelletizer and to acentrifugal dryer for drying said PET pellets, an injector to introducea pellet speed expediter into said slurry line containing said water andpellet slurry in advance of said dryer, said injector being configuredto inject said pellet speed expeditor as a gas at a velocity of at least100 m³/hour, said injector being positioned along said slurry line at apoint before any water is removed from said slurry line so that thepellet speed expediter increases a speed of said slurry to create awater vapor mist in which the water is separated from the pellets whileboth the water and pellets are still in and moving through the slurryline, and a post dryer unit to receive pellet output from said dryer andachieve crystallization of said pellets utilizing internal heat of thepellets exiting said dryer.
 15. The apparatus as claimed in claim 14,wherein the pellet speed expediter is an inert gas moving at a velocityof between 100-175 m³/hour.
 16. The apparatus as claimed in claim 14,wherein said post dryer unit is a vibration unit that keeps said pelletsin movement during said crystallization.
 17. The apparatus as claimed inclaim 14, wherein said post dryer unit is a heat insulating container.18. The apparatus as claimed in claim 14, wherein a portion of saidslurry line is straight and angled upwardly at an angle between 30° and60°, said injector being configured to inject said pellet speedexpeditor in line with said straight and upwardly angled slurry lineportion.
 19. The apparatus as claimed in claim 14, further comprising avalve mechanism downstream of said injector to further regulate saidpellet speed.
 20. An apparatus for processing PET polymers into pelletswhich comprises an underwater pelletizer to cut extruded PET polymerstrands into pellets, piping to introduce water into said underwaterpelletizer and to form a slurry line to transport a water and pelletslurry out of said underwater pelletizer and to a dryer for drying saidPET pellets, and an injector configured to introduce a pressurized, highvelocity pellet speed expediter into said water and pellet slurry insaid slurry line, said injector being positioned at an injection pointproximate to an output of the underwater pelletizer with a portion ofsaid piping between said underwater pelletizer and said injector beingclosed, said pellet speed expediter injected by said injector increasinga speed of water and pellet flow from the injection point through adownstream portion of the slurry line to the dryer where the water isremoved, the increased speed of said water and pellet flow through thedownstream portion of the slurry line causing said pellets to exit saiddryer with sufficient internal heat to initiate crystallization of saidpellets.